


Volume 51, Nº 4 (2016)
- Ano: 2016
- Artigos: 17
- URL: https://journals.rcsi.science/0015-4628/issue/view/9393
Article
Numerical modeling of downslope flows of different rheology
Resumo
Naturally occurring flow along a long homogeneous slope is numerically simulated. It is taken into account that the flow is able to capture the slope material and to entrain it into motion. The flow depth and velocity increase with time at the expense of the capture. The medium in motion is simulated using different rheological models including those of Herschel & Bulkley and Cross, as well as the power-law fluid model. For all the models the time dependences of the total depth and the mean flow velocity are obtained. The slope inclination effect on the dynamic flow parameters is studied. For the Herschel–Bulkley model the yield strength effect is also investigated. On the basis of the numerical calculations some assumptions are made and then used to derive asymptotic formulas for the bottom material entrainment rate at large times from the entrainment onset for all the above-listed rheological models.



Unsteady outflow of a warm dense Van der Vaals medium from a plane layer, a cylinder, and a sphere
Resumo
The unsteady outflow of a warm dense boiling compressiblemedium, initially at rest, from a plane layer, a cylinder, and a sphere into a vacuumis investigated in the approximation of an inviscid and non-heat-conducting two-parameter “gas-liquid”, whose thermodynamic properties are determined by the Van der Vaals equation of state. The expansion, the boiling, and the two-phase medium generation are assumed to be thermodynamically equilibrium, while the transition is instantaneous. The speed of sound suffers a discontinuity across the phase transition line of the second kind (binodal), whereas the pressure, the temperature, the density, the entropy, and the enthalpy remain continuous. The main issue in the thermodynamic calculations, which are the same for all the problems, is the construction of the binodal and, at the same time, an isentrope in an equilibrium two-phase mixture, reduced to the numerical integration of two ordinary differential equations. The one-dimensional problems of unsteady outflow are solved by means of the method of characteristics using the isentropes obtained in the thermodynamic calculations. As distinct from the plane problem, in the cylindrically and spherically symmetric problems there are no regions of homogeneous boiling liquid, which would be finite in space and time.



Hydrodynamic structure of viscous flows in rotating convergent-divergent channels
Resumo
The results of the numerical calculations of isothermal viscous flows in convergent-divergent channels axially rotating at a constant angular velocity are presented. With reference to the example of the calculations performed at Re = 2000 the difference in the flowfields in a fixed channel and that rotating at an angular velocity Ω0 = 10 s−1 in analyzed. The results of the calculations of viscous flows in convergent-divergent channels at Re = 2000 and Ω0 = 10 s−1 are also presented.



Structure of the fluid interface in an external magnetic field in the presence of a magnetizable surfactant
Resumo
The structure of the flat interface between two conventional fluids in an external magnetic field in the presence of a magnetizable surfactant is investigated with account for the dependence of the free energy of the system on the surfactant concentration gradients and the bearing phase density. The dependence of the surface tension tensor components on the magnetic field strength is determined.



Effect of crystal and crubicle rotation on the flow stability in the Czochralski model at low Prandtl numbers
Resumo
The results of a numerical investigation of the joint effect of crystal and crubicle rotation on the flow stability at low Prandtl numbers (Pr=0.01 to 0.07) are presented. The regimes with an elevated stability threshold are determined for various combinations of crystal and crubicle rotations and the heat flux distributions over the growing crystal endface are obtained. The mechanisms of the loss of stability, as the critical values of the Grashof number and the rotation velocities are exceeded, are considered and a new regime of stable nonaxisymmetric flow is established.



Interaction between periodic disturbances and a turbulent jet
Resumo
The results of a numerical and experimental investigation of the interaction between harmonic disturbances and a turbulent jet are presented. On the basis of large eddy simulation it is established that the narrow-band noise of a supersonic jet considerably increases, when the forcing amplitude amounts to thousandths and more of the total pressure of the flow within the nozzle. An analysis of the results of a laboratory experiment on the measurement of the longitudinal velocity spectra in the core of a low-velocity jet shows that the acoustic disturbances generated by a fan inside the nozzle lead to the generation of intense tonal hydrodynamic disturbances in the low-velocity jet.



Role of the pressure gradient in flows controlled by a near-wall body force
Resumo
Steady two-dimensional flows of a viscous incompressible fluid over a plane boundary in the presence of a near-wall body force are studied. The calculations are performed for a very simple one-parameter force distribution simulating the action of a single plasma actuator. The important role played by the pressure gradient in the flows controlled by body forces is shown.



Special features of the pressure fluctuation field structures in the vicinity of bluff bodies (cylinders)
Resumo
The fields of turbulent near-wall pressure fluctuations on a surface in a flow are experimentally investigated in the presence of cylinders located in the proximity of the surface. It is shown that the main characteristics of the pressure fluctuation fields can be essentially transformed in the case of nonzero distance (gap) between the cylinder and the surface in a flow, namely, the spectral levels increase over the entire frequency range, a narrow-band frequency maximum is formed, and the spatial distribution of the pressure fluctuation intensity changes. The dependence of the main parameters of the pressure fluctuation fields is presented and a weak influence of the boundary layer thickness is shown.



The effect of fine evaporating droplets on the adiabatic-wall temperature in a compressible two-phase boundary layer
Resumo
A steady-state supersonic flow of a viscous heat-conducting gas with an admixture of small droplets over a flat plate is considered. The plate surface is assumed to be thermally insulated, and its equilibrium temperature is greater than the evaporation point of the droplets. In contrast to previous publications, the case of low-inertia droplets, which do not deposit onto the wall and have time to evaporate in the boundary layer, is considered. Within the two-fluid approximation for the laminar gasdroplet boundary layer with a compressible carrier phase, a parametric numerical study of the effect of evaporating droplets on the boundary layer structure and the temperature of the adiabatic wall is performed. The similarity parameters are found and the range of these parameters is determined, in which the adiabatic-wall temperature is reduced substantially due to the droplet evaporation even for very low initial concentrations of the liquid phase. This makes promising the use of the condensed phase in the schemes of gasdynamic energy separation based on heat transfer between the flows in subsonic and supersonic boundary layers.



Detonation waves in polydisperse gas suspensions of monofuel in tubes with an abrupt expansion
Resumo
The distinctive features of detonation wave propagation in polydisperse (double-fraction) gas suspensions of a monofuel in tubes with an abrupt expansion are numerically investigated. Numerical calculations are performed for different sizes and relative mass contents of the particles of both fractions. A comparative analysis of the effect of mono- and polydisperse monofuel particles on the detonation wave mitigation is made. The dependences of the critical tube-diameter ratio of a sectional pipeline on the relative mass content and polydispersity of monofuel particles with different sizes are presented.



Investigation of hydrodynamic instability of CO2 injection into an aquifer
Resumo
The two-dimensional problem of supercritical carbon dioxide injection into an aquifer is solved. Shocks and rarefaction waves propagating in a sequence from an injection well into the formation are described within the framework of a complete nonisothermal model of flows in a porous medium. In the approximation of isothermal immiscible water and carbon dioxide flow the hydrodynamic stability of the leading displacement front is investigated for various reservoir pressures and temperatures. The parameters of unstable fronts are determined using a sufficient instability condition formulated in analytic form. The approximate analytic results are supported by the direct numerical simulation of CO2 injection using the complete model in which thermal effects and phase transitions are taken into account.



Horizontal mixing layer in shallow water flows
Resumo
Horizontal-shear thin-layer homogeneous fluid flow in the open channel is considered. A one-dimensional mathematical model of the development and evolution of the horizontal mixing layer is derived within the framework of the three-layer scheme. The steady-state solutions of the equations of motion are constructed and investigated. In particular, supercritical (subcritical)-in-average flow concepts are introduced and the problem of the mixing layer structure is solved. The proposed model is verified on the basis of comparison with a numerical solution of two-dimensional equations of shallow water theory.



Numerical modeling of the self-oscillation onset near a three-dimensional backward-facing step in a transonic flow
Resumo
The results of a numerical investigation of the nature of self-oscillation processes occurring in transonic flow past a backward-facing step and a cavity with a flow of the open type are presented. The turbulent flow past the above-mentioned bodies is modeled using the NOISEtte software package intended for solving problems of aerodynamics and aeroacoustics on unstructured grids. The modeling is performed using the eddy-resolving IDDES method that belongs to the class of hybrid RANS-LES approaches. The adequacy of the calculations is confirmed by means of comparing the results obtained with the available experimental data. The structure and the salient features of the self-oscillatory, hydrodynamic-in-nature process, which arises in flow past a cavity and a backward-facing step, are established.



Stability of gas mixture flow with a stabilized detonation wave in a plane channel with a constriction
Resumo
The stability of a flow with a stabilized detonation wave is studied within the framework of a detailed kinetic mechanism of the chemical interaction. The flow is due to the initiation of detonation combustion of a stoichiometric hydrogen-air mixture that enters into a plane channel with a constriction at a supersonic velocity greater than that of the self-sustained detonation propagation. The flow under consideration is numerically investigated using the software package developed by the authors. It is established that the flow formed in the channel, whose geometric parameters ensure the detonation stabilization in the case of the inflow Mach number M0 = 5.2, is stable against strong disturbances of a certain type. The effect of an increase in the inflow Mach number and the dustiness of the combustible gas mixture entering into the channel on the stabilization of detonation combustion in the flow is investigated.



Rarefied gas flow into a vacuum from a plane long channel closed at one end
Resumo
The time-dependent problem of rarefied gas flow into a vacuum from a plane long channel closed at one end is solved on the basis of the kinetic S-model. The effect of diffuse molecular reflection from the channel walls on the flow velocity and the process of channel cavity vacuumization is studied as a function of the channel length and the extent of gas rarefaction under the condition that the wall temperature is maintained to be constant. The kinetic equation is solved numerically using a conservative finite-difference method of the second order of accuracy in spatial coordinates. The possibility of simplification of the problem for long times by means of reduction to the diffusion process is considered.



Catalytic properties of a copper transducer for determining flow parameters in a high-frequency induction plasmatron
Resumo
The rate coefficients of the elementary stages of the complete system of heterogeneous catalytic recombination of dissociated oxygen on a copper oxide surface are determined on the basis of quantum-mechanics calculations within the framework of cluster models. The coefficients are used to calculate the dependence of the effective coefficient of heterogeneous catalytic recombination of oxygen atoms on the temperature and the partial pressure on a wide range of surface conditions. It is established that it can considerably vary depending on these conditions.



Investigation of noise generation by turbulent jets on the basis of numerical simulation of unsteady flow in mixing layers
Resumo
In the previous experimental studies it was concluded that the turbulent jet noise is produced by large-scale motions in the mixing layer induced by turbulence intermittence. The burden of this numerical simulation is the validation of these conclusions. As a result of numerical calculations, the “instantaneous” flow patterns and the parameter distributions in the initial regions of turbulent jets are obtained. On the basis of this information the flow dynamics are investigated. In the jet flow there are observable slowly transforming low static pressure regions and zones of elevated static pressure. These regions are displaced at the convection velocity. The inflow induced by the low pressure in the mixing layer has streamlines entering into the low pressure zones and flowing around the elevated pressure zones. The motion of the zones of the static pressure varying along the flow produces velocity disturbances in the induced external flow. The succession of the transformations of the intermittence-induced static pressure disturbances into sound waves is determined. This transformation occurs in the regions occupied by the ejected air.


